Title of Invention

A NOVEL PROCESS FOR CASTING OF POLYMETHYLMETHACRYLATE BASED COMPONENTS

Abstract

A novel process for casting of Poly methyl methacrylate based components, comprising of following steps: characterized in that preparing a prepolymer syrup by mixing purified methyl methacrylate monomer or a mixture of purified methyl methacrylate monomer and Polymethyl methacrylate(PMMA) in presence of purified polymerizing catalyst at a temperature in the range of 40 to 80°C for a period of 5-15 minutes, followed by heating the syrup to 92 to 98°C for the period of 5 to 15 minutes to obtain a syrup, cooling the said syrup at a temperature in a range of 0 to 30°C to obtain prepolymer, degassing the prepolymer obtained above, pouring the prepolymer slowly or step by step to a mould of desired shape kept at a temperature ranging from 35 to 60°C to give component shape, for a period depending on the final thickness of the component in the range of 10-25 mm / day followed by annealing the component in the mould at a temperature ranging form 98°C to 105°C, for a period of 6 to 48 hours and then removing the component from the mould.

Full Text

The invention particularly relates to a novel process for casting of Polymethylmethacrylate based components. Polymethylmethacrylate (PMMA) is an important engineering plastic with the exceptionally high weather resistance mechanical properties, machinability coupled with the possibility of transparency of the level of over 90%. Various methods are employed to give a desired shape to PMMA such as extrusion, injection moulding, casting, insitu polymerization of monomer, machining etc. In situ polymerization has been extensively used for preparing transparent items such as tubes, sheets, bangles etc. The process has following steps, a) Preparation of a solution containing appropriate amount of initiator in methylmethacrylate monomer. b) Activation of initiator and polymerization of monomer to a desired degree of molecular weight. In the process of polymerization 13 kcal/more of heat is generated which is very high. Reference can be made to vinyl polymerization, vol 1 part 1, Marcel Dekker, New york, 1967, edited by G.E. Ham in article "Heats of polymerization and their structural and Mechanistic Implications" by R.M. Joshi and B.J. Zwolinski. In addition to this thermal conductivity of polymeric solution decreases with the degree of polymerization which results in heat pockets in the system. These heat pockets create voids/ bubbles / microbubbles in the casting and deteroriate the transparency of the final product. Due to this problem no technology is available in the world which can be used of the development of sheet of more than 25 mm thickness, by this method. For making acrylic sheets more than 25 mm, thickness, two or more sheets of desired shape are joined together. Similarly for making a hollow cylindrical item, the available acrylic sheets are heated to their softening temperature and are bent by application of pressure. After bending, sheet is joined to form a hollow cylinder. One such item is Draft Tube Cone (D.T.Cone) for hydroturbines, which is fabricated by joining sheets of Poly methyl methacrylate (PMMA). These joints oftently open on the application of pressure during it's use. There is no method available at present for casting a seamless cone, which is necessary for making endoscopic study of the flow behavior of water in models of hydraulic turbines. The heat dissipative material such as plasticizers or additional monomer have been used as claimed by R.Donald Leonard in US Patents NO 04650821 and 04603160 wherein methyl metacrylate with 0.5 to 20% of carboxylated cellulose ester were mixed with 1-100 ppm of vanadium promoter and solution was poured and cured in the mould. Some other work has been reported in US patents no 05413870 and 05847036. None of the processes claimed so far are capable of making items having thickness more than 25 mm and with the high degree of optical transparency. In order to make transparent item having thickness more then 25 mm by insitu polymerization of (PMMA) a novel process for casting of PMMA based components has been developed. The main objective of the present invention is to provide a process for casting of PMMA based component. Another objective of the present invention is to provide a process for casting of PMMA block of desired transparency. Still another objective of the present invention is to make a casing of thickness more than 25 mm. Accordingly the present invention provides a novel process for casting of Poly methyl methacrylate based components, comprising of following steps: characterized in that preparing a prepolymer syrup by mixing purified methyl methacrylate monomer or a mixture of purified methyl methacrylate monomer and Polymethyl methacrylate(PMMA) in presence of purified polymerizing catalyst at a temperature in the range of 40 to 80°C for a period of 5-15 minutes, followed by heating the syrup to 92 to 98°C for the period of 5 to 15 minutes to obtain a syrup, cooling the said syrup at a temperature in a range of 0 to 30°C to obtain prepolymer, degassing the prepolymer obtained above, pouring the prepolymer slowly or step by step to a mould of desired shape kept at a temperature ranging from 35 to 60°C to give component shape, for a period depending on the final thickness of the component in the range of 10-25 mm / day followed by annealing the component in the mould at a temperature ranging form 98°C to 105°C. for a period of 6 to 48 hours and then removing the component from the mould. In an embodiment of the present invention purification of methylmethacrylate is carried out by conventional purification method such as alkali hydroxide washing or distillation. In another embodiment of the present invention the ratio of MMA to PMMA varies from 100 to 1. In yet another embodiment of the present invention polymerizing catalyst used is purified Azoisobutyronitrile (AIBN) or the benzoyl peroxide In still another embodiment of the present invention, degassing of prepolymer is done by applying the reduced pressure. In still another embodiment of the present invention the catalyst used is 0.01 to.03 wt% of the monomer. This process uses monomer, catalyst ranging from 0.01 to 0.03 weight %, cleaning agent such as NaOH or KOH etc. ranging from 2 to 30 weight%, water bath temperature range from 10°C to 100°C and kept uniform with in limit of +2°C This process comprises following steps; Cleaning or purification of monomer.Purification of Catalyst, Syrup Preparation .Removal of air / monomer if required, Step wise pouring of syrup into mould, Annealing of Component, Removal from mould and polishing of article, In the present invention a novel technique for pouring of syrup slowly or in steps is proposed. Approximately 15 mm thick layer of syrup is added after every 18 hours. This process is continued until the final height / thickness of product is achieved. Steps involved are detailed below. (I) Cleaning or purification of monomer Purification of chemicals used in this process is necessary because commercially available chemicals have impurities of different types, which influence the process. Monomer is stored in inhibited condition by adding some inhibitors in it. Before actual use these monomers are made free from inhibitors or cleaned. "Washing Process" is used for removing inhibitors from monomer. In this process inhibitor added monomer is washed with alkaline isolution. The solution contains alkali ranging from 2 to 10 weight percent. In the washing process example, monomer was washed with 5% NaOH solution in distilled water. The inhibitor reacts with alkali and the reaction product being water soluble separate from the monomer. The alkali solution is removed. The washing with alkali is repeated for four times or until the alkali sotution becomes colourless. Then drying agent for removing water traces from the monomer is added. Typically anhydrous Calcium Chloride crystals are put into the monomer for half an hours with shaking. (II) Purification of catalyst: Commercial available Azo bis isobutyro nitrile (AIBN) or benzoyl peroxide catalyst are not pure. They are generally, 97% pure. Further purification is necessary to obtain 100% purity of catalyst to get proper reaction. It is done by recrystallization method as described below. Catalyst is dissolved in Chloroform at room temperature until saturation occurs. It is recrystallized by adding methyl alcohol into the saturated solution in twice the quantity of Chkroiorm.lt is then filtered and dried at room temperature. It is necessary to keep it in dry place. The entire process is carried out at room temperature only. (III) Preparation of Prepolymer (syrup) from monomer: This prepolymer (syrup) was prepared by following method. Method-1. The syrup was prepared by stirring purified MMA monomer at a temperature ranging from 40°C to 80°C. Benzoyl Peroxide (purified) is then added up to 0.03 percent by weight and again stirred at 72°C for a duration ranging from 5 minutes to 15 minutes (approx.). Then solution is heated further to a 92°C temperature and stirred for 5 to 15 minutes to achieve desired viscosity. Then the syrup is quickly cooled to low temperature ranging from 0°C to 3Q°C with constant agitation to prevent 'range away'. Method-2. Purified MMA monomer and PMMA granular (polymer) are mixed together in the ratio of 4:1 for several hours till a clear, transparent and homogeneous syrup is obtained. Purified Benzoyl Peroxide is then added. Stirring of syrup continued is at 40°C until uniform solution is obtained. The syrup is again heated from 40°C to 98°C and stirred for 2-5 minutes. The syrup is cooled quickly between 0°C to 30°C with constant agitation to prevent 'run - away'. (IV) Removal of air / monomer, if required: Syrup was heated up to 45°C and vacuum is applied for removal of trapped air and unreacted monomer until prepolymer is degassed. (V) Step wise pouring of syrup: Syrup is obtained into the mould. The quantity of syrup poured in mould was adjusted so that the thickness / height of syrup is not more than 50 mm. A constant temperature ranging from 35°C to 60°C is maintained. The water temperature needs to be accurately controlled, to ensure uniform heating for polymerization to commence. After 5 to 30 hours syrup is added carefully in mould to built another layer. This process is repeated until complete desired height of the component achieved. (VI) Annealing of component: After hardening of the top most layer of block / cone / article the temperature of water is increased to 100°C. Casting is heated for 16 hours in water bath to anneal it. (VII) Removal of article from mould and polishing: After removal from the mould the article is machined to desired dimensions and is first polished by sandpaper, then by polishing paper. Thereafter cotton buffing wheel and wax polishing stone is used for final finish. Steps III, IV and V are unique and ncvel, which open possibility of making thick acrylic blocks, cones, or other large size articles. Presently, all over the world, polymer syrup is filled into closed moulds and polymerization is started. This method is useful for thin walled item or thin sheets. However, for higher thickness this method fails because of following reasons: During polymerization of MMA reduction in volume is significant. This reduction is greater than 21 percent by volume. This contraction is difficult to compensate by any method employed so far. More over, heat generated during polymerization is very large. Therefore the temperature control is difficult, which causes flaws and voids in final moldings. fhe following examples are given by way of illustration and therefore should not be construed to limit the scopes of the present invention . Example 1 Ingredients, Purified MMA monomer - 1000ml Purified Benzoyl peroxide - 0.20 g. Process. Purified MMA and benzoyl peroxide were mixed in a flask at 40°C for 5 minutes and the temperature of solution was raised to 70°C. Stir: ing was continued to keep temperature uniform for 10 minutes. The solution was heated to 95°C for a period of 5 minutes and then immediately cooled to 10°C by using water at 5°C. Then the degassing is carried out at a temperature 40°C and at a reduced pressure of 120 mm of Hg for one hour. This syrup was then poured in the rectangular mould kept in water bath at temperature 45°C for 24hrs. Solution to achieve 20mm thick layer was poured in the mould. After 24hrs another 20mm thick layer of solution was added to this semisolid solution and the process was repeated for 12 days to enable us to get a final thickness of 190 mm. Properties Transparent rectangular block of 190 mm thickness was.obtained. Example 2 Ingredients. Purified MMA monomer - 1000ml Purified Benzoyl peroxide - 0.30g Process. Purified MMA and benzoyl peroxide were mixed in a flask at 40°C for 5 minutes and the temperature of solution was raised to 60°C. Stirring was continued to keep temperature uniform for 10 minutes. The solution was heated to 92°C for 5 minutes and then immediately cooled to 10°C by using water at 5°C.. Then the degassing is carried out at a temperature 40°C and at a reduced pressure of 100 mrn of Hg for one hour. This syrup was then poured in the rectangular mould kept in water bath at temperature 55°C for 24hrs. Solution corresponding to 15 mm thick layer was poured in the mould. After 24hrs another 15 mm thick layer of solution was added to this semisolid solution and the process was repeated for 20 days to enable us to get a final thickness of 220 mm. Properties Transparent rectangular block of 220 mm thickness was obtained Example 3. Ingredients. Purified MM A monomer - 1000ml Purified Benzoyl peroxide - 0.10g. Process. Purified MMA and benzoyl peroxide were mixed in a flask at 40°C for 5 minutes and the temperature of solution was raised to 65°C. Stirring was continued to keep temperature uniform for 10 minutes. The solution was heated to 95°C for a period of 5 minutes and then immediately cooled to 10°C by using water at 5°C. Then the degassing is carried out at a temperature 40°C and at a reduced pressure of 120 mm of Hg for one hour. This syrup was then poured in the rectangular mould kept in water bath at temperature 52°C for 24hrs. Solution for only 20mm thick layer was poured in the mould. After 24hrs solution for another 20mm thick layer was added to this semi solid solution and the process was repeated for 10 days to enable us to get a final thickness of 150 mm. Properties Transparent rectangular block of 150 mm thickness was obtained. The main advantages of the present invention are 1. This process of present invention can be used for the development of seamless cone, cylinder, solid cylinders, solid blocks, rectangular shaped items etc. 2. This process of this invention is useful for developing the embedded articles. 3. This process can be used for obtaining desired thickness ranging from 1 mm to 100 mm 4. Process described can be used for the development of the components of height ranging from 1mm to 1000 mm. We Claim: 1. A novel process for casting of Poly methyl methacrylate based components, comprising of following steps: characterized in that preparing a prepolymer syrup by mixing purified methyl methacrylate monomer or a mixture of purified methyl methacrylate monomer and Polymethyl methacrylate(PMMA) in presence of purified polymerizing catalyst at a temperature in the range of 40 to 80°C for a period of 5-15 minutes, followed by heating the syrup to 92 to 98 C for the period of 5 to 15 minutes to obtain a syrup, cooling the said syrup at a temperature in a range of 0 to 30°C to obtain prepolymer, degassing the prepolymer obtained above, pouring the prepolymer slowly or step by step to a mould of desired shape kept at a temperature ranging from 35 to 60°C to give component shape, for a period depending on the final thickness of the component in the range of 10-25 mm / day followed by annealing the component in the mould at a temperature ranging form 98°C to 105°C, for a period of 6 to 48 hours and then removing the component from the mould. 2. A process as claimed in claim 1, wherein the ratio of purified methyl methacrylante monomer to polymethyl methacrylate is 100 to 1. 3. A process as claimed in claims 1 to 2, wherein the purified polymerzing catalyst is azo bis isobutyronitrile or benzoyl peroxide. 4. A process as claimed in claims 1 to 3, wherein degassing of prepolymer is done by applying the reduced pressure. 5. A process as claimed in claims 1 to 4, wherein the polymerizing catalyst used is 0.01 to .03 wt% of the monomer. 6. A process for casing of polymethyl methacrylate based components substantially as herein described with reference to the examples.

Documents:

1200-del-2001-abstract.pdf

1200-del-2001-claims.pdf

1200-DEL-2001-Correspondence-Others.pdf

1200-DEL-2001-Correspondence-PO.pdf

1200-del-2001-description (complete).pdf

1200-del-2001-form-1.pdf

1200-del-2001-form-18.pdf

1200-del-2001-form-2.pdf

1200-del-2001-form-3.pdf